1. Field of the Invention
The present invention generally relates to optical spread spectrum communication systems using frequency hopping techniques.
2. Background Information
In modern fiber optic communication systems, available bandwidth is not a pressing issue because thousands of GHz of spectrum are potentially exploitable in the optical domain. However, utilizing this vast spectrum in an optimal manner that provides both maximum capacity and maximum economic benefit is difficult to achieve. Furthermore, it is particularly expensive to transform an existing network infrastructure so that it can easily accommodate any anticipated future increases in data flow over the vast spectrum.
Wavelength division multiplexing (WDM) has emerged as the primary method by which to exploit the vast optical spectrum available in a fiber optic network. WDM is a classical method for providing multiple access by dividing the optical spectrum into fixed standardized channels centered on specific wavelengths. As additional network capacity is required, more channels must be added which increases the number and complexity of the components in the network.
To further enhance the capacity of the WDM channels, a time-division multiple access (TDMA) scheme has been employed. TDMA allows multiple users to share the same channel by assigning users with a time slot within a larger time frame. When all of the time slots for a particular channel are assigned, the absolute maximum capacity has been reached and there is no means to support additional users on that channel.
Thus, WDM and WDM/TDMA hybrid systems have a hard limit on data handling capacity which, when exceeded, corrupts the systems and disrupts the communications of all users on an overloaded channel.
In this application, an erbium doped fiber ring laser (EDFRL) is a multimode optical source used as a specific illustrative example of a tunable laser that is modified in accordance with the present invention. Alternatively, a tunable laser such as a semiconductor optical amplifier ring laser (SOARL), an external fiber cavity semiconductor laser (EFCSL) or the like may be used.
In the ring configuration of a conventional EDFRL, the allowed longitudinal modes of the laser satisfy the boundary condition:
                              cos          ⁡                      (                                                            2                  ·                  π                  ·                  f                  ·                  n                                c                            ·              x                        )                          =                  cos          ⁡                      (                                                            2                  ·                  π                  ·                  f                  ·                  n                                c                            ·                              (                                  x                  +                                      N                    ·                    L                                                  )                                      )                                              (        1        )            
where f is the optical frequency, n is the index of refraction in the fiber, c is the speed of light in vacuum, x is the positional coordinate along the circumference of the ring relative to a coordinate system, L is the effective circumference of the ring which includes the actual physical length of the fiber ring plus any additional effective lengths due to the various components inserted into the ring which cause additional delays, and N is an integer (N=0, 1, 2, 3 . . . ). Equation (1) is equivalent to writing:
                                          (                                          2                ·                π                ·                f                ·                n                            c                        )                    ·                      (                          N              ·              L                        )                          =                  2          ·          M          ·          π                                    (        2        )            where M is also an integer (M=0, 1, 2, 3 . . . ).
Letting f·N=fm where fm are the mode frequencies, then Equation (2) immediately reveals the allowed mode frequencies of the fiber laser based solely upon its physical layout:
                              f          m                =                  M          ·                      c            n                    ·                      1            L                                              (        3        )            
and the mode spacing is therefore given by:
                                          ∂                          f              m                                            ∂            M                          =                                            c              n                        ·                          1              L                                =                      f            1                                              (        4        )            The modes are equally spaced and are all harmonics of the fundamental frequency f1.
A solution based solely upon geometrical considerations allows an infinite number of longitudinal modes. The gain spectrum of the erbium fiber places finite limits on the lower and upper frequencies that can exist in the ring laser. Erbium fiber (amplifiers) can typically supply enough gain to overcome the losses in the ring within a band of wavelengths ranging from about 1520 nm through 1580 nm with the optimal band being between 1530 nm through 1560 nm. The wavelength band from 1520 nm to 1580 nm corresponds to a frequency band of 7.495 THz while the reduced band from 1530 nm through 1560 nm corresponds to a frequency range of 3.771 THz. The total number of modes that exist is given by the ratio of the amplifier bandwidth divided by f1, or more generally:
                              M          max                =                                            (                                                c                                      λ                    MIN                                                  -                                  c                                      λ                    MAX                                                              )                        ·                                          n                ·                L                            c                                =                                    (                                                1                                      λ                    MIN                                                  -                                  1                                      λ                    MAX                                                              )                        ·            L            ·                          n              .                                                          (        5        )            Accounting for the M=0 term, the total number of modes which can exist is 1+Mmax.
For a typical EDFRL with an effective circumference L of about 25 meters, the total number of modes is quite large as the mode spacing is only about 8.17 MHz (n=1.47 in fiber). Mmax=918,000 for the full erbium band and 462,000 for the reduced more optimal band. If a band restricting optical filter is inserted into the EDFRL, then the number of allowed modes is given by the reduced EDFRL bandwidth divided by f1. For a 3 GHz band pass filter in an EDFRL with an effective circumference L of about 25 meters, the number of allowed modes which can exist is thus reduced to no more than 367 with the actual number being much smaller due to the roll-off characteristics of the filter attenuating the modes that are found closest to its skirts.
When a multimode optical signal is detected by a photoreceiver with an appropriate bandwidth, the optical modes “beat” with one another producing a radio frequency (RF) comb spectrum with lines spaced f1 apart from one another starting at zero frequency and moving up in steps of f1. Either the bandwidth of the EDFRL or the photoreceiver determines the maximum observable (usable) beat frequency.
FIG. 1 shows a conventional unfiltered EDFRL configuration 100 including an erbium doped fiber amplifier (EDFA) 105, an optical isolator 110 and an optical directional coupler 115A (e.g., a fusion spliced fiber tap coupler). An optical spectrum analyzer (OSA) 120 is in communication with the coupled port of the optical directional coupler 115A via the direct port of a second optical directional coupler 115B. The OSA 120 is used to view the multimode amplified spontaneous emission (ASE) spectrum generated by the EDFRL configuration 100. A radio frequency spectrum analyzer (RF-SA) 125 is in communication with the coupled port of second optical directional coupler 115B via a photoreceiver 130. The RF-SA 125 is used to view the resulting RF comb spectrum, due to the modes beating in the photoreceiver 130.
FIG. 2 shows a conventional filtered EDFRL configuration similar to FIG. 1 with the addition of a tunable filter 205. The OSA 120 is used to view the multimode ASE spectrum and a laser line at the passband wavelength of tunable filter 205. The RF-SA 125 is used to view the resulting band limited RF comb spectrum, due to the modes beating in the photoreceiver 130.